Proposal for New Geothermal Models and Sites Hierarchy in Djibouti Republic

Abstract

An update of available geological geochemical and geophysical data, together with new field works undertaken by the Ministry of Energy, Water and Mineral Resources of the Djibouti Republic have allowed for an attempt to propose new geothermal models for several sites identified for their geothermal potential from previous general surveys and specific studies. While other contributions describe in detail the characteristics of each site, this paper aims at proposing a synthetic view of the different components and an overall concept, mainly based on the geodynamic and hydrogeological environment of the central and south-western part of the Republic of Djibouti.

Concerning the heat source, two major cases occur:
- The geothermal systems located at the top or in the immediate vicinity of the oceanic ridge, whether submarine or emerged. That is the case of Asal, Nord-Ghoubbet, MandaInakir, Rouéli and Obock.
- The geothermal systems relying on deep faulting, frequently transverse to the dominant rift system (NW-SE) without evidence of active volcanic heat sources. This is the case of Garrabayis and eventually Abhé.

For the geothermal reservoir, we also consider that it relies directly upon the geodynamic environment of the site. It may either result from simple extension and be produced by normal faulting and open fissures, as is the case in Asal, Obock and MandaInakir, or result from transverse faulting with the eventual association of block rotation, as in Nord-Ghoubbet, Rouéli, Garrabayis and Abhé.

The composition of the geothermal fluid in the reservoir will directly rely upon its hydrogeological environment. From marine brines at Asal, it will vary from meteoritic water in MandaInakir, Abhé, or Nord-Ghoubbet, to dominant sea water component in Obock and Rouéli. Continental brines will probably dominate in other sites of endoreïc basins and strong evaporation (such as Allol-Sakhalol) which have not yet been identified as geothermal sites at present due to insufficient knowledge.

As a whole, our approach helps to provide a first hierarchy of the geothermal potential of Djibouti in terms of perspective for future power production. Ridge located sites have the best heat source and therefore the largest quantitative potentials. But transverse fracturing and block rotation will allow for development of the best reservoirs, and water composition will directly influence the costs of systems and maintenance. Of course, real quantitative figures will be possibly approached only after feasibility drilling and production tests will have been undertaken. There is also need to develop an overall prospective and strategy for the country, that the present approach, further works and confrontation of experts views will help to finalize.